Yes, I noticed. But unless there is a way to get the actual MIME-type of the requested file I don't see a way to go around that. That's why in the plugin I disabled the default conversion of files unless they are on the local hard drive. By the way if you want to see your snippet in action: I published a preliminary version.
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KonstantinKDec 31 '11 at 9:24

Loic Duros has blogged about implementing nsiContentPolicy in and SDK-based addon, AFAIK this would get you the content type based functionality you'd need: lduros.net/posts/…
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canuckistaniDec 31 '11 at 23:34